Critical Mass is a bicycling event typically held on the last Friday of every month in over 300 cities around the world. While the ride was originally founded in 1992 in San Francisco with the idea of drawing attention to how unfriendly the city was to cyclists,the leaderless structure of Critical Mass makes it impossible to assign it any one specific goal. In fact, the purpose of Critical Mass is not formalized beyond the direct action of meeting at a set location and time and traveling as a group through city or town streets on bikes.

Critical Mass rides have been perceived as protest activities. A 2006 New Yorker magazine article described Critical Mass’ activity in New York City as “monthly political-protest rides”, and characterized Critical Mass as a part of a social movement; and the UK e-zine Urban75, which advertises as well as publishes photographs of the Critical Mass event in London, describes this as “the monthly protest by cyclists reclaiming the streets of London.” However, Critical Mass participants have insisted that these events should be viewed as “celebrations” and spontaneous gatherings, and not as protests or organized demonstrations. This stance allows Critical Mass to argue a legal position that its events can occur without advance notification of local police.

Critical Mass rides vary greatly in many respects, including frequency and number of participants. For example, many small cities have monthly Critical Mass rides with fewer than twenty riders which offer safety in numbersto cyclists in those locales, while on the opposite extreme, in what have been the largest events using the name Critical Mass, cyclists in Budapest, Hungary hold only two rides each year on April 22 (Earth Day) and September 22 (International Car Free Day). The ‘Budapest style’ attracts tens of thousands of riders. The April 20, 2008 Budapest ride participation was estimated at 80,000 riders.

Critical Mass differs from many other social movements in its rhizomalxerocracy (rather than hierarchical) structure. Critical Mass is sometimes called an “organized coincidence”, with no leadership or membership. The routes of some rides are decided spontaneously by whomever is currently at the front of the ride, others are decided prior to the ride by a popular vote of suggested routes often drawn up on photocopied flyers. The term was coined to describe a process by which the route for a Critical Mass can be decided: anyone who has an opinion makes their own map and distributes it to the cyclists participating in the Mass. Still other rides decide the route by consensus. The “disorganized” nature of the event allows it to largely escape clampdown by authorities who may view the rides as forms of parades or organized protest. Additionally, the movement is free from the structural costs associated with a centralized, hierarchical organization. In order for the event to function, the only requirement is a sufficient turn-out to create a “critical mass” of riders dense enough to occupy a piece of road to the exclusion of drivers of motorized vehicles. Authorities in New York, California and Oregon have expressed concern with the difficulty of coordinating with the riders, due to the lack of leadership.